The cathodic protection from corrosion of a metallic structure which is at least partially immersed in a liquid involves depressing the potential of the structure so as to inhibit the anodic process of dissolution of the metallic atoms into the liquid as metal ions. This is commonly achieved by either one of two techniques, known as the impressed current technique, in which the structure is connected to the negative terminal of a d.c. power supply, and current passes through the liquid to the structure from one or more anodes connected to the positive terminal of the power supply; and the sacrificial anode technique, in which a number of blocks of metal less noble than that of the structure are attached to, and in electrical contact with the structure at various points. In the sacrificial anode technique, the metal of the anodes corrodes in preference to the metal of the structure, which becomes the cathode in the corrosion cell in which the liquid is the electrolyte, and consequently the anodes require replacing periodically.
The invention is particularly, but again not exclusively applicable to surveying a subaqueous structure protected by a sacrificial anode protection system.
An indication of the effectiveness of a cathodic protection system may be obtained by measuring the potential of the protected structure relative to the electrolytic liquid, and if the potential is above a predetermined "safe" level the current in an impressed current system is increased, whilst in a sacrificial anode system, the anodes which have become excessively worn and are no longer able to maintain the level of current flow necessary to depress the structure's potential sufficiently are replaced.
It is common practice, therefore, to monitor the potential of the structure to ensure that adequate protection is maintained, and since the protected structures are generally of considerable size, the potentials at not only one, but a number of points about the structure are monitored to ensure that all parts are properly protected. In many structures, such as ships' hulls, this monitoring is achieved relatively simply by means of cables connected permanently to various parts of the structure, and all leading to a central monitoring apparatus. However, such permanent connection is impracticable with structures to which access is limited, such as undersea pipelines.